According to Wikipedia, the swastika was allegedly stamped on the Buddha's chest after His parinibbana. Is this true?

This symbol (right-hand) is alleged to have been stamped on The Buddha's chest by his initiates after his death. It is known as The Heart's Seal. This would predate any other particular use ascribed to it in other texts.

Or a conflation with Jainism. Jain Tirthankaras are depicted in rupas as having a Swastika in the middle of their chest. Jain and Buddhist iconography have clearly developed showing a mutual influence.

Interesting.
The main Shakyamuni rupa at Chenrezig Institute in SE Qld has a crucifix on his chest. It was explained to me that an elderly practitioner donated a rosary to the appeal for cash and kind to build the Buddha rupa. Lama Yeshe,who founded the Centre,decided to incorporate the crucifix into the statue.
kind regards

Ben

“No lists of things to be done. The day providential to itself. The hour. There is no later. This is later. All things of grace and beauty such that one holds them to one's heart have a common provenance in pain. Their birth in grief and ashes.”
- Cormac McCarthy, The Road

Learn this from the waters:
in mountain clefts and chasms,
loud gush the streamlets,
but great rivers flow silently.
- Sutta Nipata 3.725